Watches whose dial is lit using a light source to allow a user to read the time in the dark have been known for very many years. These watches differ from each other in the intensity of the lighting provided by the light source. For part of them, the light source brightly illuminates the watch dial. This can prove advantageous if a salesperson wishes to demonstrate the qualities of his product to a potential purchaser and show him the appearance the watch will have during nighttime use. The user will thus be able to see the watch display in its illuminated state, despite the lighting prevailing in the sales point. However, if the user of the watch wishes to check the time during the night, the great intensity of the lighting is likely to dazzle him. Moreover, this solution has the considerable inconvenience of consuming a large amount of energy, which constitutes a serious handicap in the case of portable electronic objects of small dimensions such as a watch whose energy storage capacities are necessarily limited. In order to overcome this drawback, it has been proposed to illuminate the watches less brightly but still sufficiently, of course, for the user of such a watch to be able to read the time-related or other information in the dark. This second solution has the main merit of being economical from the point of view of electric power consumption. However, it is practically impossible, unless one withdraws to a poorly lit place, to demonstrate the illuminating qualities of the watch at the sales point, since the illumination is too slight to be able to be seen in the light of day.
A new step was made in the state of the art by proposing, as is done, for example in U.S. Pat. No. 4,995,016 in the name of the Seikosha company, to provide the watch with a light sensor capable of detecting the various levels of intensity of the ambient lighting and adapting the illumination of the display device with which the watch is provided, as a function of the detected level of light.
A device of this type means that one no longer has to choose, during construction of the watch, between intense or low lighting, for example of the display device for the data provided by said watch. Thus, when the ambient lighting is weak (in the dark or semi-dark), lighting of the display device itself is weak, which, from the point of view of electric power consumption, is very favourable and enables the user of the watch, nonetheless to consult it at any time, particularly in the middle of the night. However, when the ambient lighting is intense, the light sensor deactivates the watch lighting means. However, when the ambient lighting is strong, the light sensor deactivates the watch lighting means. However, the watch is provided with a switch which, when it is activated, allows the watch to be brightly illuminated even in full daylight, for example in a watch boutique, in order to allow the salesperson to demonstrate the qualities of the product to a potential purchaser and show him the appearance the watch will have during night time use. The addition of an extra component in the form of a light sensor is not however without certain problems. This represents an extra cost both from the point of view of the number of components to be used and from the point of view of the assembly and manufacturing time, and introduces a new source of possible failure, which may, at more or less long term, be detrimental to the reliability of the electronic watch thereby equipped. Further, this system of detection is directive and its efficiency is a function of the location of the sensor. A shadow produced, for example, by a shirtsleeve, falsifies measurement of the degree of ambient lighting. In order to overcome these problems, those skilled in the art have no other choice than to increase the detection surface. However, this measure considerably harms the aesthetic appearance of the watch and increases its size.
It is an object of the present invention to overcome the drawbacks of the prior art in addition to others by providing a lighting device for a portable electronic object, which allows the illumination of data displayed by the electronic apparatus to be controlled reliably and inexpensively as a function of the intensity of the ambient lighting.